Catalysts are commonly coupled to engine exhaust systems for reducing regulated engine emissions. The catalysts may be configured with different coatings to promote catalyst efficiency and reduce catalyst light off time (e.g., the amount of time it takes for a catalyst to reach a predetermined efficiency). However, even with higher performance catalyst coatings, it can be important to control engine exhaust gases entering the catalyst or the catalyst efficiency may degrade.
In U.S. Pat. No. 6,591,605 catalyst efficiency may be improved by adjusting engine air-fuel ratio via feedback from a combination of a time varying signal and an output of a post catalyst oxygen sensor. However, if there is an error between the output of the post catalyst oxygen sensor and the time varying signal, a single error adjustment term simultaneously accounts for errors in amplitude, phase, and frequency. As a result, adjusting the engine air-fuel ratio for a phase error in the post catalyst oxygen sensor output may cause an undesirable disturbance in the amplitude and/or frequency of the post catalyst oxygen sensor output. Consequently, it may be somewhat difficult for the output of the post catalyst oxygen sensor to converge to the time varying signal during some operating conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for improving engine air-fuel control. One example of the present description includes a method for adjusting an air-fuel ratio of an engine, comprising: adjusting an air-fuel ratio applied to engine cylinders via a frequency adjustment and a duty cycle adjustment, the frequency and duty cycle adjustments based on a duty cycle and frequency of a signal derived from an oxygen sensor positioned downstream of a catalyst.
By adjusting an air-fuel ratio supplied to an engine via frequency and duty cycle adjustments, it may be possible for an output of a post catalyst oxygen sensor to converge to a desired response at a faster rate. In particular, when individual adjustments are made to an engine air-fuel ratio for frequency errors and/or duty cycle errors between the output of a post catalyst oxygen sensor and a predetermined signal, it may be possible to compensate for the errors with less affect on other signal attributes.
The present description may provide several advantages. Specifically, the approach may improve catalyst conversion efficiency. In addition, the approach may provide more consistent vehicle emissions since duty cycle errors can be compensated separately from frequency errors. Further, the approach provides for duty cycle and frequency adjustments for a broad range of operating conditions beyond basic engine operating conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.